Electrolytic anode

ABSTRACT

An improved anode for the electrolysis of brines is comprised of a corrosion resistant valve metal substrate, a thin porous adherent exterior coating of a refractory oxide which is inert to the electrolysis cell environment, and between the substrate and exterior coating a thin layer of ruthenium oxide.

United States Patent Keith et al. Apr. 18, 1972 [54] ELECTROLYTIC ANODE 721 Inventors: Carl 0. Keith, Summit; Alfred J. Haley, [56] Merems Cm g F a Robert M Cran- FOREIGN PATENTS OR APPLICATIONS or a o 1 6,660,302 11/1966 Netherlands "204/290 [73] Ass1gnee: Engelhard Minerals 8: Chemicals Corpora- Primary Examiner-Daniel E. Wyman Assistant Examiner-l. Vaughn [22] Flled' I969 Attorney-Miriam W. Lefi and Samuel Kahn [2i] Appl. No: 880,932

57 ABSTRACT Related US. Application Data I An improved anode for the electrolysis of brmes is comprised commuatlon'm'pal'l of 786,438, of a corrosion resistant valve metal substrate, a thin porous ad- 1968 herent exterior coating of a refractory oxide which is inert to the electrolysis cell environment, and between the substrate Ls. F and exterior coating a layer of ruthenium oxide [51] Int. Cl ..B0lk3/04 [58] Field of Search .204/290 F 5 Claims, No Drawings the electrolysis of an aqueous solution of sodium chloride. in

such cells graphite anodes are usually used commercially. Although the graphite anodes are not entirely satisfactory because their wear rates are high and impurities suchas C:

are introduced in the products, 'no satisfactory substitutes have yet been found.

Platinum group metal coated electrolytic valve metals have been proposed as substitutes for graphite anodes. These metallic anodes offer several potential advantages over the conventional graphite anodes, for example, low'er overvoltage, lower erosion rates, and higher purity products. The economic advantages gained from such anodes, however, must besufficiently 'high to overcome the high cost of these metallic anodes. Anodes proposed heretofore have not satisfied this condition. Therefore commercialization of the platinum group metal anodes has been limited.

One problem is the life of the metallic anodes. A factor which contributes to shortening the anode life is the so-called undercuttingeffect. For economic reasons the precious metal coatings are very thin films so that exposure of the substrate is imminent. This is particularly true in the use of low overvoltage coatings which are inherently porous. Although corrosion resistant, the valve metals are attacked through the pores of these coatings thereby shortening the life of the anodes.

Another problem is the loss of precious metal during operation of 'the cell. Although the loss is, gradual, it is costly because the precious metals are expensive and because the erosion of the thin coatingshortens the anode life. The loss of precious metal may be from mechanical wear. At' the high current densities desirable in commercial installations, the increased rate of flow and the excessive gassing is conducive to such mechanical wear. In mercury cells a contributing factor is amalgamation of the precious metals.

A further problem in mercury cells is "shorting" of the cell on contact of the precious metal with the mercury with consequent effects, such as amalgamation, change in the surface of the anodes with resultant harmful change in electrolytic properties, and cell stoppage.

A still further consideration which is of major importance in the highly competitive manufacturing processes involving the electrolysis of brines is the power consumption associated with the anodes. Power costs represent a substantial percentage of the total production costs and even a small reduction in power consumption produces a material economicadvantage.

It was an object of this invention to provide metallic anodes with improved physical and electrical characteristics. it was a further object to provide a process for the electrolysis of brines which can be effected with materially lower production costs.

In accordance with this invention the electrolysis of brines can be effected with a materially lower power consumption. This is achieved by the use of an improved anode. The anode not onlyreduces the power consumption in the cell, but also it has been found to have long life and low metal losses due to mechanical wear and amalgamation. The resistance to amalgamation makes the anode particularly useful in mercury cells.

The anode of the present invention is comprised of a corrosion resistant metal substrate, a ruthenium oxide coating, and over the ruthenium oxide coating a thin porous adherent coating of a refractory oxide, which is inert to the electrolysis cell environment. Examples of suitable refractory oxides are oxides of Si, Ti, Ta, Nb, Hf, Zr, W, Al, and combinations thereof.

The refractory oxide coating is porous. It is particularly important with respect to the exterior silica coating that it has a high surfacearea.

The corrosion resistant metal substrates, the so-called valve metals, used for electrolytic anodes are well known in the field. They are much less expensive than platinum group metals and they have properties which render them corrosion resistant to the anodic environments in electrolysis cells. Examples ofsuitable corrosion resistant valvemetals are Ti, Ta, Nb, Hf, Zr, W, Al, and alloys thereof. it is also-well known to have the valve metal as a layer on a base metal such as copper which is a'good electrical conductor but corrosive to the environment, and such modifications are within the scope of this invention.

The refractory oxide coating not only minimizes the contact of the precious metal layer with the electrolyte, but also minimizes penetration of the electrolyte to the valve metal and thus limitsthe extent of undercutting effects. Another advantage is that it minimizes shorting" and the concomitant problems. Surprisingly however, despite the dielectric characteristics of the exterior coating, these advantages are gained without sacrificing the desirable electrical properties of the precious metal anodes. Indeed, the exterior porous high surface area silica coating improves the electrolytic properties of the thin precious metal coatings. A still further advantage of the anodes of this invention is that the high surface area porous exterior coating is conducive to gas evolution. The refractory oxide coating is also useful in improving the mechanical coherence of the ruthenium oxide deposits applied in relatively great thicknesses.

Theanodes of this invention are prepared by first forming a ruthenium oxide layer on the base metal substrate and then and cleaning the surface of the base metal, ruthenium metal or a ruthenium salt is deposited on the substrate and then the coated substrate is subjected to elevated temperatures in an oxidizing atmosphere. The ruthenium metal or salt is deposited in a variety of well known ways, e.g., the ruthenium metal may be deposited as a finely divided dispersion in an organic vehicle or by plating, sputtering, vacuum deposition, the ruthenium salt may be deposited by applying such salt dispersed or dissolved in an organic or aqueous medium. The conversion to the oxide is then effected by firing the coating in an oxygen-containing atmosphere, e.g., air, at a temperature above-about 400 C. The firing time depends on the temperature, oxidizing atmosphere used, and the thickness of the ruthenium metal coating applied. Typically a suitable ruthenium oxide is formed by firing the metal film in air at 500 C. for about five minutes, however, temperatures in the range of 400 to l,000 C. and even higher may be used.

The exterior high surface area porous refractory oxide coating is deposited from a dispersion or solution containing a precursor compound of the refractory oxide or the refractory suitable to deposit the refractory oxide coatings from formulations containing a compound of the refractory metal in an organic vehicle which formulations develop into the refractory oxides on firing. It has also been found particularly suitable to deposit the porous exterior coatings from a dispersion or solution containing a hydrophilic refractory oxide in very fine particle size, e.g., having a surface area of at least 30 square meters per gram (m /g). The coatings are fired at temperatures of greater than about 400 C. to promote bonding. When fired at temperatures lower than about 400 C. the coatings are not sufficiently adherent. Preferred methods involve depositing the refractory oxide from an aqueous colloidal solution or from an organic resinate formulation. Preferred temperatures forforming an adherent coating are 600 to l,0O0 C. and

higher, e.g., suitable coatings have been formed at l,200 C.

With respect to the silica coating, it is particularly important Oxides of Si, Ti, and Ta have been found particularly suitable. that it has a high surface area. Coatings formed from colloidal hydrophilic silica are found to be adherent, porous, and have high surface area.

More than one coating of the refractory oxide may be applied. Generally, the refractory oxide coatings are effective at a thickness of up to about 200 microinches. Thicker coatings are often not sufficiently porous. Alternatively, multiple thin coatings may be formed by depositing alternate layers of ruthenium oxide and refractory oxide, thereby forming a hard durable multilayer coating on the substrate. The temperature range for firing the successive alternate layers is in the range of about 400 C. to l,000 C., and even higher temperatures may be used. It is not required that all layers be fired at the same temperature. Generally, however, the final layer is preferably fired at a temperature above 800 C. Coatings of improved adherence and uniformity can be formed, for example, at temperatures of l,200 C. without unduly sacrificing the electrical characteristics of the anode. The electrical properties vary with the crystalline structure of the Ti substrate; grades of Ti, e.g., having greater yield strength, are more re sistant to oxidation during high temperature firing in air. They also vary with the degree of cold rolling given the Ti prior to coating and with the thickness of the RuO coatings. Although the multilayer coatings are effective at thicknesses of over 200 microinches, there is no advantage in forming thicker coatings because of their durability even when exceedingly thin.

The following examples are given by way of illustration and not as a limitation of the invention. It will be appreciated that modifications within the scope and spirit of the invention will occur to those skilled in the art.

Examples 1, 2 and 3 show comparative tests in diaphragm and mercury electrolysis cells using various anodes. For each anode a sheet of commercially pure titanium, k in. X 3 in. X 0.063 in., is prepared for coating by etching in concentrated hydrochloric acid for a period of 18 hours at room temperature and cleaning in fluoboric acid. In these three examples the coatings are prepared as described below.

RuO coatings are prepared as follows:

An aqueous solution of RuCl (containing 10.35 percent by weight of Ru) is applied to one side of a titanium sheet using a brush. Successive coats are applied, each being fired at 500 C. in air for five minutes until a coating of the desired thickness is obtained. Alternatively a ruthenium resinate solution (containing 4 percent by weight Ru) is applied. In still another alternative method an alcohol based paint is used. This paint is composed of lg of RuCl:,, 1 ml. of linalool and 30 ml of 2-propanol. X-ray diffraction analysis of samples similarly prepared, by firing the deposited coating in air at the indicated conditions, showed that a major portion of the ruthenium was converted to ruthenium oxide.

Porous adherent silica coatings are prepared as follows:

After forming the Ru layer, it is overcoated with SiO by applying a formulation containing hydrophilic colloidal silica. Ludox HS, an aqueous colloidal silica solution, is used in the formulations. The formulations contain about percent colloidal silica and 90 percent water. Film forming additives such as sodium titanate, silicate or borate may be incorporated in minor amounts in the colloidal silica solution. For example, suitable coatings are made from a formulation composed of 10 percent colloidal silica, 0.5 percent sodium titanate and 85.5 percent water. Successive coats of silica are applied and fired in air at 500 C. for 5 minutes until a coating of the desired thickness is obtained.

The thickness of the coatings is determined gravimetrically.

EXAMPLE 1 Two samples are prepared having a RuO, coating equivalent to 17 microinches of Ru metal on a titanium substrate.

Sample B is used as prepared.

Sample A is overcoated with 100 microinches of SiO using the method described above. The silica has a surface area of about 70 m /g.

Sample A and Sample B are used as anodes in a laboratory scale diaphragm cell for the electrolysis of 25% NaCl solution. The tests are run at a temperature of 35 C. and a current density of 1,000 amperes per square foot (ASF). The chlorine overvoltage is determined with a conventional Luggin capillary probe, and the results are set forth in Table I.

After 210 hours, Sample 8 would not draw the specified current density at its initial cell potential. Upon raising the cell potential rapid disintegration of both the coating and the substrate resulted.

This example demonstrates the superior electrical and wear properties of the anode having the SiO: exterior coating of this invention over an anode having a RuO layer and no overcoating of silica.

EXAMPLE 2 Samples similar to those described in Example 1 are prepared. Sample C is a titanium substrate with a RuO, coating having a thickness equivalent to 17 microinches of Ru. Sample D is a titanium substrate with a RuO, layer equivalent to 17 microinches of Ru and microinches overcoating of silica. Each of the samples is masked with pressure tape so that an area of 0.049 in of coating remains exposed. Samples C and D are then used as anodes in a small cell using a mercury pool as the cathode and a 25% NaCl solution as the electrolyte. The anodes are subjected to a mercury shorting test as follows:

The exposure area of the test coating is allowed to generate chlorine at 1,000 ASF in the brine and then it is submerged in the mercury pool and the change in current density is measured. The tests show that Sample D, having the RuO layer and the SiO exterior coating is very much less susceptible to shorting than the same coating without the protective exterior coating of SiO It will be appreciated that since the resistance to shorting is higher the anodes of this invention may be positioned in closer spacial relationship with a mercury cathode without danger of shorting and with concomitant lower power requirements.

EXAMPLE 3 Samples similar to those described in Example 1 are prepared, except that the Ru0 layer is thinner. Two samples are repaired each having a RuO coating equivalent to 2 microinches of Ru on a titanium substrate.

Sample E is used as prepared.

Sample F is overcoated with microinches of SiO, using the method described above.

Samples E and F are used as anodes in a laboratory scale diaphragm cell and tested for chlorine overvoltage using the procedure described in Example 1. The cell using Sample F, the anode in accordance with this invention, has an initial chlorine overvoltage of 220 millivolts and a cell potential of 4.30 volts. The cell using Sample E as the anode shows erratic behavior. The coating of Sample E is poorly adherent and the EXAMPLE 4 Two sheets of commercially pure titanium, in. X 3 in. 0.063 in., are prepared for coating by sandblasting the surfaces with aluminum oxide grit followed by cleaning with an abrasive cleanser. Both sheets are then coated on both sides with a formulation composed of (by weight) 11.5 percent ruthenium chloride, 42.3 percent 2-propanol, and 46.2 percent linalool. The coated substrates are heated to 300 to 400 C. for l to 2 minutes and then fired at 500 C. for 5 minutes in an open air furnace to form a Ru coating.

Sample G is prepared by repeating the application of the ruthenium formulation and heat treatment twice, so that a total of three coats of ruthenium oxide are applied.

Sample H is prepared by overcoating the first ruthenium oxide coating with a porous silica coating. The porous silica coating is formed by applying an aqueous colloidal silica solution composed of (by weight) 31.6 percent Ludox HS (containing 30% SiO 0.5 percent sodium titanate powder, and 67.9 percent water. The silica-coated substrate is heated to 500 C. for minutes. Thereafter the procedure of applying and firing alternate coatings of ruthenium oxide and silica is repeated twice.

The composition of the samples is as follows:

Coating Sample 0 Sample H M0 100% 43.3% SiO, 0 56.7%

The gravimetric weight gain in both samples is equivalent to 6.1 microinches of Ru metal. The weight of SiO in Sample H is equivalent to 58 microinches of SiO Samples G and H are used as anodes in a laboratory scale diaphragm cell and tested for chlorine overvoltage using the procedure described in Example 1. Sample G having 3 coatings of ruthenium oxide-has an initial chlorine overvoltage of 155 millivolts and a cell potential of 4.20 volts, Sample H, a multilayer RuO -SiO coating prepared in accordance with the present invention, has an initial chlorine overvoltage of millivolts and a cell potential of 4.30 volts. In addition the multilayer RuO -SiO coating, applied in alternate layers is more adherent than the Ru0 coating of Sample C.

EXAMPLE 5 Sample I and J are prepared as follows:

Two sheets of commercially pure titanium l in. X 3 in. x 0.040 in. are prepared for coating in a manner similar to that described in Example 4. Approximately 5 microinches of ruthenium oxide are formed on both sides of these substrates from five applications of a formulation composed of 6 percent ruthenium chloride, 44 percent 2propanol and 50 percent linalool, with firing after each application to form a ruthenium oxide coating. The firing conditions for both samples is 500 C. for 10 minutes in air but Sample J is thereafter fired at l,200 C. for 30 seconds in air. Observation of the color showed both samples to be mainly ruthenium oxide.

Both samples were then coated with approximately 100 microinches of SiO deposited as described in Example 4, except that Sample .1 is subjected finally to a temperature of l,200 C. in air for 30 seconds.

In order to test the adherence of the coatings the following simple tape test is performed: Each sample is weighed to an accuracy of about 0.1 mg. White-gummed electrical tape is firmly pressed to both sides of the samples and then removed. This operation is repeated an additional two times, with clean tape used each time. The samples are then rinsed in acetone, scoured, rinsed in hot water and dried in a stream of hot air. Thereafter the samples are reweighed and the amount of coating removed determined by difference. The weight record of Samples 1 and J are shown below:

Samples 1 .l Wt of RuO, on 1' x 3" 0.0074 0.0078 Wt of SiO, 0.0250 0.0257 Wt of composite coating 0.0324 0.0335 Wt of coating removed in tape test 0.0045 0.0013 Wt of coating remaining after tape test 89.2% 96.7%

The results of this test show that the coatings formed by firing at the higher temperatures, e.g., 1,200 C., have improved adherence.

The overvoltage characteristics of these coated anodes depends, in part on the thickness of the intermediate. ruthenium oxide layer. Anodes having a ruthenium oxide coating of, for example, 40 microinches and a porous exterior silica coating fired at high temperatures, e.g., 1,200 C., as described above, havev not only excellent adherence but also excellent electrical properties.

This example not only illustrates a method of preparing the RuO and SiO,, coating by depositing alternate layers of RuO, and SiO but also further demonstrates the improved physical and electrical properties of anodes of this invention.

EXAMPLE 6 Sample K is prepared as follows:

A sheet of commercial pure titanium 1 in. X 3 in. X 0.040 in. is prepared by sandblasting the surface, in a manner similar to that described in Example 4. The titanium sheet is then fired at 500 C. in air for 10 minutes to produce a surface coating of a conductive oxide of titanium. The purpose of forming this coating-is to improve the spreading of the aqueous ruthenium salt solution.

An aqueous solution containing, by weight, 25 percent ruthenium chloride (40% Ru) in water is used to apply approximately 4 microinches of ruthenium in one application. The coated substrate is fired in air at 700 C. for 5 minutes to form a ruthenium oxide coating.

A commercially available tantalum resinate formulation, sold by Engelhard Minerals & Chemicals Corp. as resinate No. 7,522, is then applied over the ruthenium oxide layer and fired in air at 650 C. for 5 minutes. The tantalum resinate paint, fired in this way, deposits a layer of tantalum oxide. Two applications of the tantalum resinate are applied, each being fired separately, to deposit a tantalum oxide coating of approximately 10 microinches.

The application of the one coat of ruthenium oxide followed by the deposit of two coats of tantalum oxide is repeated three times, forming a total deposit of ruthenium oxide in all the layers of equivalent to approximately 16 microinches of ruthenium.

Sample K is compared with a low overvoltage anode having a 40 microinch coating of Pt-Ir on a titanium substrate in a laboratory scale diaphragm cell as described in Example 1. Sample K has a cell potential of 1.350 volts and an anode potential of 5.50 volts. The low overvoltage reference anode has a cell potential of 1.240 volts and an anode potential of 5.50 volts.

Sample K is cut in l in. X l in. sections and one section is used as an anode in a diaphragm type laboratory scale chlorine cell at 1,000 ASF for hours. The change in thickness after use in the cell is determined by the difference in thickness of the coating used as the anode and an untested l in. X l in. section, measured by X-ray fluorescence. After 110 hours of use the section shows a decrease in thickness of 0.2 microinches. This wear rate is calculated to be 0.041 grams of ruthenium lost per ton of chlorine produced.

EXAMPLE 7 Sample L is prepared as follows:

A sheet of commercially pure titanium is prepared as described in Example 6 with a thin surface coating of titanium oxide developed on the substrate by firing in air. A ruthenium oxide layer is developed as described in Example 6, except that 3 coats of the ruthenium chloride formulation are applied and fired to give a ruthenium oxide coating equivalent to approximately l2 microinches of ruthenium. The superficial coherence of this coating is poor.

Two coats of a titanium formulation containing 7.2 percent titanium (sold by Engelhard Minerals & Chemicals Corp. as titanium resinate No. 9428) are applied, each being fired in air at 650 C. for 5 minutes giving an estimated gravimetric thickness of about 6-10 microinches of titanium oxide. The coherence of Sample L is excellent.

On evaluating Sample L and a 40 microinch Pt-Ir low overvoltage reference anode in a chlorine cell, as described above, Sample L shows electrical properties comparable to the low overvoltage reference anode:

Thickness of the Ruthenium Oxide Coating Anode Cell as Ru Potential Potential Sample (microinches) (volts) (volts) L 12 1.260 5.80 Ref. 40 1.210 5.75

The titanium oxide exterior coating not only improves the adherence and coherence of the ruthenium oxide but also the electrical properties of the titania coated anode are comparable to a conventional low overvoltage anode. The coating is less susceptible to mercury amalgamation than a conventional precious metal coating and it has longer life.

We claim:

1. An anode for the electrolysis of brines comprising a corrosion resistant valve metal substrate, a thin adherent porous exterior coating of a refractory oxide, said refractory oxide being inert to the electrolysis environment, and between the substrate and the exterior coating a thin layer of ruthenium oxide.

2. The anode of claim 1 wherein the exterior refractory oxide coating has a thickness of less than 200 microinches.

3. The anode of claim 1 wherein the refractory oxide coating is an oxide of at least one element selected from the group Ti, Ta, Nb, Hf, Zr, W, and Al.

4. The anode of claim 3 wherein the refractory oxide is titania.

5. The anode of claim 3 wherein the refractory oxide is tantalum oxide. 

2. The anode of claim 1 wherein the exterior refractory oxide coating has a thickness of less than 200 microinches.
 3. The anode of claim 1 wherein the refractory oxide coating is an oxide of at least one element selected from the group Ti, Ta, Nb, Hf, Zr, W, and Al.
 4. The anode of claim 3 wherein the refractory oxide is titania.
 5. The anode of claim 3 wherein the refractory oxide is tantalum oxide. 